This group works on gene regulation in the immune system. During this period we have worked on the function of ICSBP, a transcription factor that belongs to the interferon regulatory factor (IRF). Previous work showed that ICSBP acts as a repressor for a number of interferon (IFN) inducible genes that carry the ISRE DNA element in their promoter. In more recent efforts using ICSBP-/- mice, we found that ICSBP has a critical role in host defense. By studying parasite infection model, the defect in ICSBP-/- mice was traced to the impairment of macrophage function, particularly their inability to induce a cytokine IL-12. This cytokine is very important for host defense, as it stimulates the production of IFN( and IL-2, which exert anti-viral/batericidal activities and stimulate T cell proliferation, leading to the activation of specific immune responses. The IL-12 P40 gene is induced in macrophages upon stimulation by many pathogens in immunocompetent hosts Based on the results from -/- mice, we have further investigated the role for ICSBP in IL-12p40 regulation The IL-12 promoter is well conserved in mouse and humans, and contains several cis regulatory elements, which appear to be involved in IL-12 p40 transcription. In transfection analysis IL-12 p40-luciferase reporters are activated by bacterial lipopolysaccharides, LPS and IFN( in macrophages. However, we found that ectopic expression of ICSBP results in marked enhancement of IL-12p40 reporter activity in a ICSBP does dependent manner and even in the absence of LPS and IFN( stimulation. These stimuli further enhanced reporter activity in ICSBP+/+ cells. The ability to stimulate IL-12 reporter activity was unique to ICSBP, since no other members of the IRF family tested stimulated reporter activity. Detailed deletion analysis of the 5= flanking region of the Il-12 promoter led us to conclude that ICSBP acts primarily through the ets site. By DNA affinity binding assays we show that the endogenous ICSBP expressed in macrophages bound to the ets site, presumably as a complex with other proteins. These results show that ICSBP is a transcriptional activator for IL-12p40, that acts by binding to a unique site in cooperation with other nuclear proteins.